1. Field of the Invention
The present invention relates to an array substrate for a flat-panel display device in which surge-protection switch circuits are connected to the display section.
2. Description of the Related Art
Recently, various apparatuses such as a personal computer, a word processor, a television, a video projector, and the like employ a flat-panel display device represented by a Liquid Crystal Display (LCD), because of its characteristics such as thin, light and low power consumption. Particularly, active matrix LCDs are intensively researched and developed. Since the active matrix LCD has Thin Film Transistors (TFTs) to drive the pixel electrodes, respectively, an excellent display image can be obtained which has no closstalk between adjacent pixels.
The structure of a typical active matrix LCD will be briefly described below. This LCD has a liquid crystal composition held between an array substrate and a counter substrate via orientation films, and displays an image by means of a light transmitted through the liquid crystal composition. The array substrate includes a plurality of pixel electrodes made of Indium Tin Oxide (ITO) and arrayed in a matrix form on a glass substrate, a plurality of scan lines formed along the rows of the pixel electrodes, a plurality of signal lines formed along the columns of the pixel electrodes, and a plurality of pixel TFTs formed near intersections of the scan lines and the signal lines. Each of the pixel TFTs is responsive to a selection signal from the scan line and supplies a pixel signal voltage from the signal line to a corresponding pixel electrode. The array substrate further includes a plurality of storage capacitance lines each of which is formed substantially in parallel with the scan lines and insulated from corresponding pixel electrodes by means of an insulating film so as to constitute a storage capacitance Cs between the storage capacitance line and the corresponding pixel electrode. The counter substrate has a matrix light-shutting film formed on the glass substrate and a common electrode formed on an insulating film covering the light-Shutting film. The light-shutting film shuts off a light transmitted through an area which is located between the pixel electrodes and the scan and signal lines, and shuts off an incident light to the pixel TFTs on the array substrate. The common electrode is electrically connected to a common-potential line provided on the array substrate by a transfer member, which is formed by dispersing electroconductive grains of silver or the like in a resin. The common-potential line, the signal lines, and the scan lines are electrically connected to a driver circuit formed on an external circuit substrate by a Flexible Print Circuit (FPC) wiring plate which has a metal wirings on a flexible base film of polyimide, or by a Tape Automated Bonding (TAB) wiring plate which has driving elements additionally formed on the FPC wiring plate. The array substrate further includes a plurality of connection pads serving as conductive terminals for receiving voltages supplied from the driver circuit to the signal lines and the scan lines, and a plurality of test pads serving as conductive terminals for receiving test voltages supplied for inspecting defects of the pixel TFTs and the wirings thereof. Since the test pads are not used except for the time of inspection, these test pads are located outside the connection pads on the array substrate, so that they can be removed after inspection has been carried out during the manufacture of the array substrate.
For example, Jpn. Pat. Appln. KOKAI Publication No. 3-296725 discloses a technique of protecting the pixel TFTs from electrostatic charge produced during the manufacture of the array substrate. According to the technique, a plurality of surge-protection switch circuits are connected between a short-circuit line formed along the periphery of the array substrate and the scanning and signal lines. Each of the surge-protection switch circuits is formed of diodes, TFTs, or the like of non-linear elements, and serves as a high resistance when a difference between the potentials of the signal or scan line and the short-circuit line is relatively small and as a conductor when the difference is significantly large. Therefore, if the signal or scan line has increased to a high potential due to electrostatic charge produced during the manufacture of the array substrate, the surge-protection switch circuit discharges the electrostatic charge from the signal or scan line to the short-circuit line. If a difference between the potentials of the gate and source of a pixel TFT has extremely increased, the short-circuit line is electrically connected to both the scanning and signal lines connected to the gate and source of the pixel TFT, thereby decreasing the difference. Accordingly, the pixel TFT is prevented from being destroyed due to an increase in the difference between the potentials of the gate and drain of the pixel TFT. At the time of inspecting defects of the pixel TFTs and their wirings, the short-circuit line is electrically disconnected from the scanning and signal lines to which test voltages are supplied. Therefore, defect inspection is not adversely affected by the surge-protection switch circuits.
Conventionally, there is a case where a plurality of surge-protection switch circuits are electrostatically destroyed during the manufacture of the array substrate, thereby short-circuiting adjacent wiring lines (signal lines or scan lines) via the short-circuit line. However, it is difficult to distinguish this short-circuit from the short-circuit caused when the wiring lines are provided in contact with each other.